1. Field of the Invention
The present invention relates to an apparatus for blowing a fragile cable such as an optical fiber or a bundle of optical fibers to insert into a tube while riding on air flow, and a method therefor.
2. Description of the Related Art
There is a method in which a cable is inserted into a tube by air flow with the cable being fed into the tube while air flow being introduced simultaneously with the feeding. In this case, the feeding of the cable into the tube at a speed higher than a cable insertion speed by air flow requires a longer insertion time, because insertion resistance is increased due to the meandering of the cable within the tube. If the cable meanders and such meandering is aggravated, the insertion of the cable is no longer possible.
EP 0 253 636 has proposed a method and apparatus to overcome this problem. In this method and apparatus, the thrust of a cable is controlled by arranging a curved section on a part of the passage and detecting the displacing of the cable toward the outer curve of the curved section at the curved section so that the application of such a force as to buckle the cable is detected.
Abnormality in cables detected by such conventional method and apparatus will be described with reference to FIGS. 5A and 5B. In FIGS. 5A and 5B, reference numeral 1 represents a cable; and 16, an abnormality detecting tube. FIG. 5A shows a normally inserted condition of a cable, in which the speed at which the cable is inserted by air flow is higher than the speed at which the cable is fed. Under this condition, the cable 1 passes along the inner curve of the abnormality detecting tube 16 as a passage. Under an abnormal condition, the speed at which the cable is inserted by air flow is lower than the speed at which the cable is fed, so that the cable moves while passing along the outer curve of the abnormality detecting tube 16 as a passage as shown in FIG. 5B.
However, under the condition in which the cable 1 is only moved toward the outer curve, the cable 1 is merely curved so as to depict a mild curve along the abnormality detecting tube 16, and this mild curve is not such as to mechanically damage the cable 1. If the feed speed is controlled to be reduced at this instance, the insertion speed is reduced more than necessary, which hence imposes the problem of increasing the total insertion time.
Namely, the curve of the cable is allowed to some degree in which the cable is not damaged. However, the conventional method and apparatus as disclosed in EP 0 253 636 does not allow such a curve.
Moreover, in a method of using the curved section for detecting abnormality, the cable 1 is inevitably worn while coming in contact with the wall surface of the abnormality detecting tube 16, thus producing resistance. Therefore, part of the insertion force is lost, which is a disadvantage.